Biological and chemical indicators used to test and/or determine the efficacy of sterilization processes are well known in the art. Typically, such biological indicators contain microbial spores which are exposed to a selected sterilant or sterilizing process. The survival of any exposed spores is determined by placing the exposed spores in an environment capable of sustaining germination of spores and subsequent growth of the germinated spores. In view of the fact that microbial spores are accepted as being much more resistant to sterilization processes than most other forms of microorganisms, it is assumed that a sterilization process that will kill microbial spores will also kill any other contaminating microorganisms. See, e.g., Disinfection, Sterilization, and Preservation, Fourth Edition, ed. Block, Seymour S., Lea & Febiger, Chapter 6 (1991) that reports general criteria needed to analyze or assess sterilization processes.
Although cell viability after exposure to a variety of sterilants and/or sterilization processes is a traditional measure of sterility, the effects of such sterilants or sterilization processes on other types of microbial activity have also been investigated. For example, enzymatic activity as described in U.S. Pat. No. 5,252,484 (Matner et al.) and germination rate as described in U.S. Ser. No. 08/196,312 and International Patent Application No. PCT/US95/01984 have also been used to determine efficacy of sterilization processes.
Since sterilization is not uniform throughout a load, the results of spore survival obtained in the least accessible part of a load must be used to determine whether the load was completely sterilized. It is common practice to place only one or two units per point at several different locations of a load of material to be sterilized. The fewer the number of biological indicator units at any one location in a sterilization process, the greater the chance for an erroneous readout of sterilization effectiveness whenever the spore survival is below 100% (based on the number of biological indicator units showing spore outgrowth). The risk of a false readout increases when spore growth periods shorter than seven days are used. Biological indicators are typically tested for accuracy of their readout results in sample sizes about 10 to 200 units per location in a sterilization process for validation purposes in order to attain a confidence level of at least 95 percent. Currently, it is not practical to use this many individual biological indicators per location in a routine sterilization process, particularly since the biological indicators need to be placed in more than one location.
There remains a need for a sterility indicator which will provide a high level of confidence that all parameters necessary to achieve sterilization, including the interrelated parameters of time, temperature and concentrations of moisture, chemicals or radiation dose, have been reached.